In a mass spectrometer, an ionized measurement sample (sample gas) is mass analyzed at a mass spectrometry section. While the mass spectrometry section is housed in a vacuum chamber and kept at a high vacuum of 0.1 Pa or less, an ionization of the sample gas is performed by a method to be ionized at atmospheric pressure as described in Patent Document 1 or by a method to be ionized in a reduced pressure of about 10 to 100 Pa as described in Patent Document 2. Accordingly, there is a difference between a pressure under an environment for performing the ionization and a pressure under an environment for performing the mass spectrometry. Therefore, a differential pumping scheme as described in Patent Document 3 has been proposed in order to introduce the ionized sample gas into the mass spectrometry section while keeping a degree of vacuum (pressure) in the mass spectrometry section within a range at which mass spectrometry is possible. In Patent Document 4, a scheme of introducing intermittently the ionized sample gas into the mass spectrometry section has been proposed in addition to the differential pumping scheme.